Gunmen in north-west Pakistan have attacked a group of Shia students, killing at least four in a sectarian attack, officials and witnesses say.
The students were walking to school in the Orakzai tribal region when they were fired upon by suspected militants.
A local tribal force later clashed with the suspected militants. Witnesses said more villagers were joining the militia and the battle was escalating.
Sunni militants have carried out frequent attacks on minority Shias.
Sunni Muslims account for around 80% of Pakistan's population and are the dominant group in the tribal areas, although Orakzai has significant Shia numbers.
The BBC's M Ilyas Khan in Islamabad says that hardline Sunni Taliban consider Shias heretics.
The attack took place near a secondary school in the Utmankhel area, near the town of Kalaya.
Witnesses said that the local tribal militia subsequently chased the militants and engaged them in battle.
As the clash intensified there were reports that heavy weapons were being used against the insurgents.
Local residents say that hundreds of militants have hideouts in the hills around Kalaya town.
The area has served as a base for Hakimullah Mehsud, a militant commander from South Waziristan who has recently replaced Baitullah Mehsud as the head of Pakistan's Taliban.